A variety of known pumps are used to dispense medical fluids. Syringes are widely used to dispense relatively small volumes of medical fluids, which can include highly concentrated medication. The maximum volume of syringes is typically about 60 ml. After this volume is dispensed, a caregiver must replace the depleted syringe to continue medication. Accordingly, syringes do not lend themselves to large volume applications such as the dispensing of large volumes of blood in various circumstances or the dispensing of high volumes of fluid, such as saline, to burn patients for example.
Syringes can be used in conjunction with syringe pumps that automatically operate the single plunger or piston of the syringe. Typically, the plunger tip is made of a soft, compliant rubber. When the plunger is pushed to dispense fluid, the tip is compressed and forced to the outer wall of the syringe. “Stiction” can then occur when the piston is moved again after being stationary, where “stiction” is a term known in the art derived from the ability to stick in combination with static and dynamic friction. In such an intermittent operation, the force required to overcome the “stiction” and start the piston moving can cause a bolus of fluid to be dispensed initially, which is undesirable.
Known pumps that are used in systems to dispense large volumes of medical fluids include peristaltic pumps, various diaphragm pumps, and single piston pumps. Although each type has been successfully used, they are subject to certain design and/or application challenges. For example, since the fluid flow passage in peristaltic pumps is normally open, fluid can be inadvertently supplied to the patient. This can occur if the tubing leading from a source of fluid, such as an IV bag, to the inlet portion of the pump is not clamped. Also, the continuous compression of the tubing defining the normally open flow path can result in tube fatigue, thereby necessitating replacement of the tube that adds to the operational cost of the system.
Peristaltic pumps are also affected by the hydraulic head height, resulting from the position of the source of fluid above the pump, which can result in further inaccuracies with the flow rate of the pump.
Large volume single piston pumps are known but do not exhibit fluid flow constancy. This is because a “dead time” occurs, for each pumping cycle, after the piston pumps a predetermined volume of fluid and then the output valve is closed, the piston is retracted and the piston chamber is refilled with fluid. This lack of flow constancy is undesirable since the half-life of certain medications can be on the order of seconds. If the medical fluid isn't delivered to and absorbed by the patient within this time, the medical fluid may be ineffective for its intended use. Flow constancy is a particularly important consideration when high potency medical fluids are being dispensed.
Known diaphragm pumps used in large volume medical fluid dispensing systems include those having a single elastomeric diaphragm and an associated piston to deform the diaphragm and dispense the medical fluid. Diaphragm pumps of this type can also include elastomeric check valves that communicate with the pump inlet and outlet ports. The compliant nature of these check valves can lead to variations in the breaking pressure of the valves, i.e., the pressures required to open or close the valves, which in turn can result in flowrate accuracy issues. A lack in flow constancy due to fluctuations in flowrate of the medical fluid being delivered is undesirable for the same reasons discussed previously with respect to the lack of flow constancy caused by “dead time.” Another challenge associated with pumps having elastomeric diaphragms, is that the diaphragm(s) deform during the fill cycle and store potential energy. This energy is released during the pumping cycle, which can cause a bolus of fluid to be dispensed initially. This temporary spike in fluid flowrate also adversely affects flow constancy and is therefore undesirable.
Another known diaphragm pump used to dispense large volumes of medical fluids includes two elastomeric diaphragms that are pumped in alternating fashion. This pump does not include elastomeric check valves and the associated challenges. However, as with the single piston diaphragm pump, the compliant, elastomeric diaphragms are pressurized during the fluid fill cycle causing them to deform and store energy. Accordingly, when the corresponding output valve is opened at the beginning of a pumping cycle, a bolus of fluid can be dispensed, even without the associated piston moving, which is undesirable.
Another challenge associated with known large volume medical fluid pumps in general is the susceptibility to air bubbles forming in the fluid system and the typical requirement of caregiver intervention to “prime” the pump to eliminate the undesirable air bubbles. Air bubbles can be formed in the fluid delivery systems associated with the pumps due to pump cavitation or “outgassing” that can occur when the temperature of the fluid is raised. Once air bubbles are detected in the delivery system the pump typically shuts down and triggers an alarm advising a caregiver of a problem. The time it takes for the caregiver to remedy the problem results in an interruption in the delivery of medical fluid to the patient. Spurious alarms result in a waste of caregiver time as well as an interruption in fluid delivery to the patient.
Yet another challenge associated with medical fluid pumps is the requirement to replace the portion of the pump that is exposed to the fluid after a predetermined, relatively short period of time as a result of hospital procedures associated with infection control. This replacement must be accomplished in an expeditious and cost effective manner.
It is therefore desirable to provide a pump having a replaceable pump module for use in medical fluid dispensing systems, which can be used in small and large volume fluid applications and overcomes the disadvantages associated with known pumps used in medical fluid dispensing systems.